This X-ray image of the galactic center merges all of Swift’s observations from 2006 to 2013. Sagittarius A* is in the center. Low energy X-rays (300 to 1500 MeV) are shown in red. Green has medium energy (1500 to 3000 MeV). High energy blue (3,000 to 10,000 eV). Credit: NASA/Swift/In. designer

An international team of researchers led by graduate student Alexis Andres found that Dungeon At the center of our galaxy, Sagittarius A*, shines not only irregularly from day to day, but also in the long term. The team analyzed 15 years of data to reach this conclusion. Andrés started researching in 2019 when he was a summer student at the University of Amsterdam. In the years that followed, he continued his research, now to be published in Monthly Notices of the Royal Astronomical Society.

Arc A* is a powerful source of radio, X-rays, and gamma rays (visible light is blocked by interfering gas and dust). Astronomers have known for decades that Sagittarius A* flickers every day, emitting bursts of radiation ten to a hundred times brighter than normal signals seen from a black hole.

To learn more about these mysterious flares, a team of astronomers led by Andres searched for patterns in 15 years of data made available by Andres. NASA Neil Geirels Swift Observatory, an Earth-orbiting satellite dedicated to detecting gamma-ray bursts. The Swift Observatory has been observing gamma rays from the black hole since 2006. Analysis of the data showed high levels of activity between 2006 and 2008, with a sharp decline in activity over the next four years. After 2012, the frequency of outbreaks increased again; the researchers had difficulty discerning the pattern.

In the coming years, the team of astronomers hopes to collect enough data to be able to rule out whether the differences in the Sagittarius A* eruptions are caused by passing gas clouds or stars, or whether something else could explain the irregular activity observed from the central black hole in our galaxy.

“The Swift Observatory’s extensive data set did not come about by chance,” says Andrés, co-author and former supervisor, Dr. Nathalie Degenaar, also from the University of Amsterdam. Her request for these specific measurements of the Swift satellite was approved when she was a Ph.D. student. “Since then, I have regularly requested more monitoring time. It is a very special monitoring program that allows us to do a lot of research.”

Co-authored with Dr. Jacob van den Igenden, or University of Oxford, commenting on the team’s findings: “It’s still not entirely clear how eruptions occur. It was previously believed that more flares occur after gas clouds or stars pass through the black hole, but there is no evidence for this yet. We cannot yet confirm the hypothesis that the magnetic properties of the surrounding gas also play a role.”

Reference: “A Rapid Study of Long-Term Changes in the X-ray Burning Properties of Arc A” by A Andrés, J van den Eijnden, N Degenaar, PA Evans, K Chatterjee, M Reynolds, JM Miller, J Kennea, R Wijnands, S Markoff, D Altamirano, CO Heinke, A Bahramian, and G Ponti, D Haggard, 9 Dec 2021, Available here. Monthly Notices of the Royal Astronomical Society.